1. Field of the Invention
The present invention relates to a fixing device favorably for use in an electrophotographic image forming apparatus, and to an image forming apparatus having the same.
2. Description of the Related Art
In an image formation using an electrophotographic system, a photoreceptor charged with a uniform electric potential is exposed to light in accordance with image information so that an electrostatic latent image is formed. The formed electrostatic latent image is developed by a developer so as to be visualized. The visualized image is transferred on a recording paper or the like, and the transferred developer on the recording paper is made to be fixed so as to form a solid recording image.
The fixing device used for such image formation, is generally composed of a heating roller and a pressure roller. The fixing device has such a configuration that, in passing the recording paper on which the developer for forming a visualized image through a pressure contact region (hereinafter referred to as a nip section) of the heating roller and the pressure roller, which pressure contact region is formed by pressing the pressure roller against the heating roller, unfixed developer is fused and fixed by heating of the heating roller and pressing of the pressure roller.
During a fixing operation in the fixing device, there sometimes occurs a so-called hot offset that the developer fused on the nip section of the both rollers is not all fixed on the recording paper, but a part of the developer is attached to a surface of the roller. For instance, the developer attached to the heating roller is transferred on a portion which should be properly a white base, on a recording paper on which the developer is to be subsequently fixed, with the result that an image defect is made to occur.
Moreover, on the pressure roller, the developer which has already fixed to a back surface of the conveyed recording paper, for instance as in a case of duplex print, is sometimes fused again by heat in passing through the nip section and a part of the developer is transferred and attached to the pressure roller. The developer thus attached to the pressure roller may cause the image defect and further, may cause a soil of the back surface of the recording paper.
The image defect caused by the hot offset in the fixing device sometimes remains, in a case of black-and-white print, mere defects such as a fog in a white base of the formed image, a soil on the back surface of the recording paper, or the like in a tolerable range. However, in a case of full-color print, since a developer having a color different from a prescribed one is transferred from the both rollers, there often occur practically intolerable defects.
As a related art for solving such a problem, there is an apparatus having roller cleaning units on the both rollers provided in a fixing device (refer to Japanese Unexamined Patent Publication JP-A 2003-107952).
FIG. 10 is a schematic view showing a configuration of a roller cleaning unit 1 provided in a related art fixing device. FIG. 10 illustrates the roller cleaning unit 1 provided on a heating roller 2 in the fixing device.
The roller cleaning unit 1 comprises a feeding roller 4 for feeding a belt-shaped cleaning member 3 which has been previously rolled up, a winding roller 5 for taking up the cleaning member 3 fed from the feeding roller 4, and a pressure-contact roller 6 (also referred to as a web pressure-contact roller) provided between the feeding roller 4 and the winding roller 5 so as to press the cleaning member 3 on the heating roller 2.
The roller cleaning unit 1 presses the cleaning member 3 on the heating roller 2 which is rotated in an arrow sign 7 direction in a state where the winding roller 5, the feeding roller 4, and the pressure-contact roller 6 are made to be at rest without being rotated so that the heating roller 2 and the cleaning member 3 are made to be slidingly scrubbed with each other. By so doing, a developer 8a attached in a fused state to an outer circumferential surface of the heating roller 2 is removed and the removed developer 8b is accumulated, still in a substantially fused state, in a gap formed by the cleaning member 3 located between the pressure-contact roller 6 and the winding roller 5, and the surface of the heating roller 2.
When the developer 8b accumulated in the gap is excessively built up, cleaning capability is decreased. Accordingly, when the accumulating developer 8b reaches a certain amount level, the roller cleaning unit 1 rotates the feeding roller 4 in an arrow sign 10 direction so as to feed the cleaning member 3, and the winding roller 6 in an arrow sign 11 direction, and further operates the winding roller 5 for take-up in an arrow sign 9 direction so as to take up the cleaning member 3, with the result that the developer 8b is made to be detached from the surface of the heating roller 2 in a state where the developer 8b is attached to the cleaning member 3.
When a feed amount of the cleaning member 3 due to this feeding roller 4 is small, a portion on the cleaning member 3 which has been once used for cleaning cannot be fully replaced by a clean portion on the cleaning member 3. Accordingly, the cleaning capability cannot be sufficiently recovered despite feeding of the cleaning member 3. Further, when the feed amount of the cleaning member 3 is too large, the clean cleaning member 3 will be unnecessarily consumed, so that running cost will be increased.
Furthermore, when a take-up amount due to the winding roller 5 is smaller than the feed amount of the cleaning member 3 due to the feeding roller 4, the fed cleaning member 3 sags between the feeding roller 4 and the winding roller 5 so that sufficient cleaning capability cannot be achieved. On the other hand, the take-up amount due to the winding roller 5 is larger than the feed amount of the cleaning member 3 due to the feeding roller 4, excessive tension is loaded on the cleaning member 3 and ultimately, the cleaning member 3 may be torn apart.
As described above, it is extremely important in the roller cleaning unit 1 to control the feed amount and take-up amount of the cleaning member 3. However, these points are not disclosed or suggested at all in the Japanese Unexamined Patent Publication JP-A 2003-107952.
In view of this problem, the applicant of the invention proposes to control each rotary operation of the feeding roller and winding roller in the fixing device so that a sum of a feed amount of the cleaning member which is fed by the feeding roller at one feeding occasion, and a take-up amount of the cleaning member which is taken up by the winding roller at one take-up occasion, is equal to or more than a double of a length of a nip section of a pressure-contact roller.
FIGS. 11A and 11B are schematic views each showing a fixing roller cleaning unit 20 provided in the fixing device in connection with the proposed art. FIG. 11A shows the fixing roller cleaning unit 20 in an initial state that a cleaning member 23 fully rolled up by a feeding roller 21 starts to be fed. FIG. 11B shows the fixing roller cleaning unit 20 in a state that almost all the cleaning member 23 previously rolled up by the feeding roller 21 has been fed, and taken up by a winding roller 22.
In the fixing roller cleaning unit 20 each shown in FIGS. 11A and 11B, the cleaning member 23 is fed from the feeding roller 21, and taken up by the winding roller 22 through a nip section 26 between a pressure-contact roller 25 disposed so as to contact a heating roller 24, and the heating roller 24 of fixing rollers. By so doing, a developer on a surface of the heating roller 24 is cleaned away.
In this fixing roller cleaning unit 20, when the surface of the heating roller 24 is cleaned, a control portion (not shown) provided in the fixing device controls each rotational angle of the feeding roller 21 and the winding roller 22 by adjusting a rotational time thereof so that a sum of a feed amount of the cleaning member 23 which is fed by the feeding roller 21 at one feeding occasion, and a take-up amount of the cleaning member 23 which is taken up by the winding roller 22 at one take-up occasion, is equal to or more than a double of a length of the nip section 26 in a circumferential direction of the pressure-contact roller 25.
Accordingly, in the fixing roller cleaning unit 20, both of the feeding roller 21 and the winding roller 22 need driving systems and control systems for controlling the rotational angle, that is to say, the rotational time. Moreover, in order to stop the rollers at exact rotary portions after the rollers rotate for a predetermined time, the feeding roller 21 is provided with a feeding roller brake 27 while the winding roller 22 is provided with a winding roller brake 28. Consequently, the apparatus has a complex configuration, and a setting space for the apparatus becomes large. This problem should be solved to improve the apparatus.